Conputer-driven display terminals using cathode ray tubes (CRT) can be categorized into the main types, the directed beam type in which the CRT beam is swept across the screen, and the raster-scan type in which the image is formed by raster-scanning the CRT beam across the screen. Typical directed beam displays are the IBM (Registered Trade Mark) 2250 and 3250 display terminals. The raster-scan type can be further sub-divided into those employing a full bit-per-pel refresh buffer and those employing a character buffer and an associated character/cell generator.
The advantages and disadvantages (such as cost, degree and speed of interaction, etc) of these different types of displays are reviewed in the article entitled "A Cell Organized Raster Display For Line Drawings" by B. W. Jordan, Jr. and R. C. Barrett in Communications of the ACM, Volume 17, Number 2, February 1974 at pages 70 to 77. This article describes in some detail a raster scan display employing a character buffer and a character/cell generator in which a display can be built up from a number of characters or symbols. As indicated in the article, the more complicated the picture to be displayed, the more characters or symbols are required to build the picture and the larger the character/cell generator. The article describes an arrangement in which the size of the character/cell generator is limited by employing a set of patterns in a read only store of the character/cell generator and manipulating (by translation, reflection and masking) these patterns to derive other patterns. This has the disadvantage of complicating the refresh logic of the display.